Is this legal?

I would like to know why and how U. S. Cellular representatives have talked customers such as myself into upgrading and purchasing new phones from them, knowing full well that these devices were going to be useless in a few short months? I've purchased 2 new phones from us cellular in the last 5 months... I had heard rumors of the future sprint buyout and spoke with the us cellular agent about it... She confirmed the rumors with me.... Yet still sold me 2 new galaxy's... Never mentioning the the devices weren't going to be compatible with Sprint... and I basically handed us cellular a good amount of money for nothing! Maybe that was my own fault for not researching the facts more before the purchase... However I do think any merchandise sales made by Us Cellular in the last 6 months or more is at the very least unethical and in my opinion criminal. Even more disturbing... I was just on the phone with a Us Cellular agent less than two weeks ago concerning a bill question... and she tried to talk me into using my 'points' to move up an upgrade for my son to purchase a new phone with them!! This was only two weeks before I recieved the letter that my new devices will soon be useless!!! Like I said unethical.... Basically theft and attempt at more theft.... How is this a legal business practice??!?!